The present invention relates to a process for the recovery of gold and more particularly to a process for recovering gold from gold-plated materials by dissolving the gold in an etchant solution and then precipitating the gold therefrom.
Gold films are widely used in the electronics industry in integrated and thin film circuitry because of their excellent electrically conductive properties. Because of gold's monetary value, it is desirable to recover the excess gold removed from electronic circuitry during etching operations as well as to recover the gold from scrap parts including gold plated printed circuit boards and other decorative or functional gold coated metal, plastic, glass, and ceramic parts.
In the past, commercial processes for dissolving gold have utilized either hot aqua regia (a mixture of concentrated hydrochloric and nitric acids) or cyanide solutions. However, both of these materials are extremely dangerous to handle. Aqua regia is highly corrosive and emits toxic fumes while cyanide solutions can emit deadly cyanide gases if proper handling techniques are not followed. Moreover, there are problems of safely disposing of these materials.
In the search for less toxic materials which will still dissolve gold, others have turned to the use of halogen or halide salt solutions. For example, Harrison, U.S. Pat. No. 2,304,823, teaches a process for recovering precious metals including gold and silver from ore by adding a solution containing iodine, potassium iodide, and nitric acid to the ore to dissolve the precious metals. Mercury is then added which forms an amalgam with the metals which is then precipitated from solution and recovered. Fink, U.S. Pat. No. 2,283,198, teaches several processes for recovering gold from ore including the use of chlorine in combination with sodium bromide and a large excess of sodium chloride or bromine to dissolve the gold. It is taught that the gold in solution can be precipitated and recovered by the addition of zinc dust or ferrous sulfate.
Jacobs, U.S. Pat. No. 3,625,674, teaches a gold recovery process in which gold ore is finely ground and then dissolved in an aqueous solution of ethanol and iodine. Gold is recovered by heating the solution to a temperature which evaporates the alcoholic solution and drives of the iodine as vapor. Bazilevsky et al, U.S. Pat. No. 3,495,976, teach a process for recovering gold from a plated substrate by dissolving the gold in an aqueous solution of potassium iodide and iodine. Sulfuric acid is then added to the solution which is heated to a temperature sufficient to drive off iodine as vapor but not evaporate the solution. Gold precipitates from the solution and is recovered as iodine is driven off.
Homick et al, U.S. Pat. No. 3,957,505, teach a gold recovery process which includes dissolving the gold in an aqueous solution of iodine and alkali metal iodide and then precipitating gold from solution by the addition of either an acid or base in combination with a reducing agent. It is further taught that the iodine-iodide solution can be regenerated by the addition of an oxidizing agent. Lastly, Wilson, U.S. Pat. No. 3,709,681, teaches a process for recovering noble metals including the steps of finely dividing ore containing the metals, dissolving the metals in an aqueous solution of a ketone, an elemental halogen, and a halide salt, and recovering the metals by the addition of heat and zinc, copper, iron, or aluminum metal to precipitate the noble metals.
However, all of the above processes require the presence of reagents in addition to the halogen or halide salts or fail to provide complete recovery of gold. Accordingly, the need still exists in the art for a simple and effective method for quantitative recovery of gold from various substrates.